1. Field of the Invention
The present invention relates to a polishing compound which does not remain stain on the surface of work piece used for the surface and edge polishing of the semiconductor substrate such as silicon wafer, further relates to an edge and surface polishing method by said polishing compound.
2. Description of the Prior Art
Electric parts such as IC, LSI, or VLSI which are made from semiconductor materials e.g. silicon mono crystalline are produced as follows. That is, an ingot of silicon mono crystalline or other compound semiconductor is sliced to a thin wafer, precise electrical circuit pattern is formed on the surface of water and divided to chips. Then, IC, LSI, or VLSI are prepared based on the chip. An as cut wafer which is prepared by slicing an ingot is lapped, etched and polished as to processed to a mirror finished wafer at least one side of which is mirror finished. As the holding method of a wafer at the wafer polishing process, a grasping method, a vacuum attracting method and a holding down method by wind pressure can be used and a tool which meets to each methods can be applied.
As the abrasives to be used for the polishing, a fine particle of an oxide of metal such as silicon, aluminum, cerium, zirconium or manganese can be mentioned. Especially, as an abrasive for the polishing of silicon wafer, a fine particle of silicon oxide is preferably used. Said particle of silicon oxide is dispersed into alkaline aqueous solution by colloidal state and forms a stable polishing compound. The term of silicon wafer in this invention indicates a basic bare wafer, an epitaxial wafer, a wafer which is covered by an oxide or a nitrated film, SOI wafer which is covered by an insulated film and a semiconductor wafer for CMP (chemical mechanical polishing).
At the polishing procedure, lined, speckled or striped whity stains are grown on the surface of tools and wafer when the polished surface is dried up, which not only hurts markedly the appearance of a mirror finished wafer, but also deteriorate the yield of the product. To avoid said problem, the additional washing procedure is needed, or a special treatment for example to keep a wafer wet is needed. The mentioned phenomenon is especially remarkable at the edge polishing procedure of silicone wafer or the surface polishing procedure of semiconductor device, and the solving of said problem had been strongly desired.
In general, the edge polishing procedure of this invention is carried out by following method. Namely, on the surface of a rotatable dram of edge polishing machine, polishing pad such as resin foam, synthetic leather or nonwoven cloth are wound and stuck, the edge part of silicon wafer whose edge is chambered is pressed to the rotating drum with necessary angle by rotating with constant supply of aqueous solution of polishing compound composed mainly by colloidal silica. Thus the polishing of the outermost periphery edge of a wafer is carried out. As a tool to hold the wafer in this edge polishing procedure, a grasping tool which simply press or nip the wafer or a vacuum attracting tool are mainly used. In the case of this edge polishing method, growth of a stain on a wafer at the point or surrounded area which said tools contact is pointed out, further, the stain grows along with the shape of the vacuum attracting tool.
And, at the polishing of an ordinary wafer (brand wafer) or a semiconductor device wafer, a slurry composed of ordinary loose abrasive is used as abrasives and a polishing machine possessing a rotating platen on which polishing pad made of resin foam, synthetic leather or nonwoven cloth is stuck is used. In this case, a wafer is fixed to a pressure plate by a vacuum attracting method, by a sticking method along with the shape of carrier ring or by a method to press the wafer to polishing pad by high pressure air, and in these methods the problem of stain growth at the contact point of tool is pointed out too. At the edge polishing and also at the surface polishing, it is necessary to extend the washing time or to use excess washing water to avoid the growth of stain.
In general, as the abrasives which is used for silicon wafer polishing, fine particles of silicon oxide can be mentioned. As the fine particles of silicon oxide, several kinds of colloidal silica can be mentioned. For example, colloidal silica which prepared by removing sodium from aqueous solution of sodium silicate by ion transferring resin and growing up as the colloid size silicon oxide fine particles, colloidal silica prepared by hydroxide of alkyl silicate such as ethyl silicate or methyl silicate and growing up as the colloid size silicon oxide fine particles and colloidal silica obtained by dispersing the fumed silica produced from silicon tetrachloride into water can be mentioned. To the water dispersion of these colloidal silica, adequate amount of additives such as alkaline component, buffering agent or dispersing agent are added, and the obtained solution is generally used as the polishing compound for silicon wafer.
The polishing method which uses the polishing compound of above mentioned composition is quite different from the mechanical process which uses for example diamond grinding stone, or alumina type abrasives. This method utilizes the chemical action of alkali to the silicon wafer, concretely utilizes the corrosive action of alkali to the silicon wafer. That is, the thin corroded layer is formed by the corrosive action of alkali on the surface of silicon wafer and said layer is removed by the mechanical rubbing action of fine particles of colloidal silica, which is so called mechano-chemical action. The polishing is proceeded by the chemical action which alkaline component of the solution has, therefore it is necessary to maintain pH of polishing compound solution bigger than 7 region. When pH of the solution is closer to 7 (neutral), the chemical action to the silicon wafer is reduced and also the polishing speed is deteriorate, on the contrary, when pH is closed to the strong alkaline region of over than 10 the chemical action becomes strong and the polishing speed becomes faster.
In such kind of polishing compound, the fine particles of silicon oxide are dispersed in aqueous solution, and the solution of polishing compound is easily dried up by heat accompanied with the processing or by the contact with dried air, and fine particles of silicon oxide gel remains on the surface of a wafer and appears as the whity stain. Further, small portion of wafer and fine particle of silicon oxide are solved in alkaline component, and when the polishing compound is partially remained on the surface of polished wafer, the dried up products of them acts as an etchant (solve promotion agent) and causes a speckled type stain.